Now switch to the alpha mask of your paint software. Here is an example as it may look in Photoshop:

Paint in the Alpha Mask using a brush with no feather to mask out unwanted areas. If you do everything right, the unwanted areas will have a red overlay:

Create a new image with the same size. Cut out a usable area from the original image and place it in the area where the unwanted areas are in the above image. Don't forget to create an appropriate alpha-mask. The new image may look like this:

Don't forget: The areas must have sufficient overlap. Otherwise enblend cannot work correctly.